Large text files and searching text

So far so good.
Please see last portion of code for what I still need to do.

void CMyDlg::OnLbnSelchangeList1()
{
  int nSelect;
  nSelect = c_List1.GetCurSel();
  CString cSelect;
  c_List1.GetText( nSelect, cSelect );

  CString JobFile;
  JobFile = _T("C:\\MyFolder\\"); //re-apply main part of
original path
  JobFile += cSelect; //add filename selected
  JobFile += _T(".txt"); //re-apply file extension

  CString mess;
  mess.Format(_T("Would you like to load \"%s\" as top ?"), cSelect);
  int a = AfxMessageBox(mess, MB_ICONQUESTION | MB_YESNO);
  if(a != IDYES)
    return;
  CFile in;

  if(!in.Open(JobFile, CFile::modeRead)){
    DWORD err = ::GetLastError();
    CString msg;
    msg.Format(_T("Error opening file: %d"), err);
    AfxMessageBox(msg);
    return;
  }

  //read entire file into string
  //search string for a "keyword"
  //copy x bytes from this point forward to another string
}

Please advise how to do the 3 things I need above.
The text file can be as large as 100mb and the copied portion can be as
large as 10mb.
Thanks,